Factors affecting the growth of bacterial colonies on agar plates

Abstract

A study has been made of the physical chemistry of the development of colonies on the surface of a minimal agar medium by a standard strain of Aerobacter (Klebsiella) aerogenes and by drug-resistant strains derived from it. Histograms of final colony diameter, constructed from measurements on plates having different numbers of colonies, have one maximum and are approximately symmetrical. Normal Gauss curves have been verified to fit some of the histograms, most of which are of similar form, although occasionally a somewhat wide spread on the side of increasing diameter occurred. When moderate or large numbers of colonies are present the total colony area supportable by the plate is proportional to the total capacity of the plate. The distribution of sizes, however, is shown to depend partly on the variation in the lags of the single cells inoculated and upon the free space left around the different individuals on the plates. A complex interplay of individual with cooperative or competitive factors thus exists. A detailed study has been made of the growth of single colonies. The initial exponential growth of the single cell is soon replaced by a phase in which the radius of the developing colony is linearly related to the time of incubation. In this phase diffusion of nutrients, buffer or waste products is not rate determining and the colonies remain more or less disk-shaped with a fairly regular perimeter. With moderate or large numbers of colonies exhaustion of the plate stops growth at this stage, but in the protracted growth which occurs when a few colonies only are present the ‘radial law’ phase gives way eventually to a slower phase in which the area of the colony increases linearly with time. Diffusion, chiefly affecting pH rather than nutrient supply, now becomes more and more the dominant factor controlling growth and the colony perimeter becomes increasingly irregular. Morphological changes may now appear. This is dealt with briefly as a prelude to a further paper. Kinetic models have been constructed for various aspects of the research and have been tested against the results.

The Growth and Respiration of Bacterial Colonies

“…cloacae and 37 pm for B. cereus. These results are similar to the 40pm calculated as the depth of oxygen penetration into growing colonies of E. coli by Pirt (1967), using growth kinetics determined in liquid culture. However, for S. albus a much lower value of g p m was obtained for the depth of oxygen penetration into colonies.…”

“…Pirt explained the discrepancy in terms of diffusion of nutrients to the population: only at the edge of the colony are substrate and oxygen present in excess together. Cooper, Dean & Hinshelwood (1968) showed that in very old colonies linear increase in radius gives way to a linear increase in area and this is explained as being due to removal of nutrient from the agar in front of the expanding colony, an interpretation that seems to fit the observations of Rieck, Palumbo & Witter (1973), who measured the concentration of glucose in the agar used to grow Pseudomonasfluorescens. Palumbo et al (1971), regarding the colony (also of Pseudomonas fluorescens) as domeshaped like a slice taken off a regular sphere, showed that colony volume increases exponentially over the first day's growth, although the viable count does not.…”

Autoradiographic Study of the Localization and Evolution of Growth Zones in Bacterial Colonies

“…Thus, Pirt (1967), Cooper et al (1968) and Palumbo et al (1971) followed growth by measuring the increase in colony diameter, and occasionally thickness, and demonstrated the presence of a clearly localized peripheral growth zone. The bacteria responsible for the increase in thickness of the colony cannot, however, be located in this zone.…”

Evaluation of plate count methods for determination of maximum specific growth rate in mixed microbial communities, and its possible application for diversity assessment

“…The transition to a linear radial growth phase, which is dependent on the organism and its physiochemical environment, has, for some organisms, been shown to occur at a colony diameter of approximately 0·2-0·3 mm (Plomley 1959;Cooper et al 1968). In general terms, this suggests that Equations 1 and 2 are applicable for colonies of a diameter ¾0·2 mm (cf.…”

“…The variation in size depends partly on the variation in the lag periods of the different sub-populations inoculated and partly on the free space around the different colonies on the plates (Cooper et al 1968). Furthermore, different numbers of cells may have given rise to each colony, as some bacteria will be present as single cells in the inoculum and others, as paired cells, etc.…”